How do I apply maps to a retopologized low poly model?
First, you need to do the retopology to create a low-poly model and uv unwrap it. Then bake the maps from the textured high-poly model to the low-poly model. Finally, connect the baked maps to the corresponding slots of the Principled BSDF node in Blender.
Use the Displacement map in the material nodes or use it with a Displace and Subsurf modifiers on the high-poly model (as you did). Then bake the Normal map. This will transfer/fake the details on the low-poly object. (Alternatively, you could bake the depth info of the Displacement map and use the baked image as a Height/Bump map. But quality might be worse.)
Note: Gloss maps are inverted Roughness maps. Use this map with an Invert node in the Roughness slot of the Principled BSDF.
I hope this helps.